The present invention relates to switching mode power supplies and more particularly to start circuits for starting the generation of pulse width modulated switching pulses.
Switching mode power supplies for converting a first DC voltage to a second DC voltage are commonly used to power electronic systems from an alternating current voltage source. Such power supplies are generally used because of their relatively low cost and because they can operate at frequencies above 20 kHZ, thereby enabling the use of much smaller and lighter power transformers, coils, and filter capacitors. The control circuits for such power supplies usually include a pulse generator or oscillator which generates a series of pulse width modulated pulses. These pulses are used to control the duration of applications of the DC input voltage across the power transformer in a single ended power supply.
In general, a conventional switching power supply includes a DC voltage source which is normally derived from an alternating current line via a rectifier, a power transformer including a primary winding and at least one secondary winding, a pulse generator for generating a train of pulses of varying pulse width as a function of the present value of the output voltage, and a transistor switch in series between the DC voltage source and the primary winding of the power transformer. This transistor switch is controlled by the pulses generated by the pulse generator such that the switch closes for the duration of each said pulse. The pulse generator monitors the output DC voltage generated by the power supply, compares this voltage to a fixed reference voltage, and either expands the pulse width of the pulse generator feedback pulse to raise the output voltage, or narrow the pulse width of this pulse to generate a lower DC output, to thereby maintain the output DC voltage at a prescribed predetermined voltage level. A rectifier and filter circuit is also connected to the secondary winding for generating the output DC voltage. Such feedback of the output DC voltage is necessary since otherwise the output voltage would vary as a function of the varying demand of the load being powered by this output voltage.
The two standard types of switching power supplies are boost (step-up) and buck (step-down) power supplies. The conventional boost switching power supply is also called a flyback power supply or flyback converter. In such power supplies, energy is stored in the power transformer when the power switch is on and then delivered from the transformer out to the load when the switch is off. More specifically, when the transistor power switch is conducting, current increases at a linear rate through the primary of the power transformer, which behaves like an inductor by storing energy in its core. As soon as the switch is cut off, the flux in the transformer core decreases, permitting the current to flow in the secondary circuit. This current charges an output capacitor as well as feeds power to the output load. The pulse width modulating pulse generator compares the output voltage with a fixed reference voltage for generation of the feedback pulse width modulated pulse.
Conventional buck power supplies include forward converter power supplies which operate in a similar manner to flyback power supplies except that a separate inductor on the secondary side of the power transformer is used to store energy rather than the power transformer. In this case, when the switching transistor is on, as current flows through the primary winding, current is also caused to flow from the secondary winding through a diode rectifier into an inductor and out to the output load. When the transistor is off, this inductor continues to provide current flow to the output load.
Both types of above described power supplies generally require some sort of start circuit to initiate the pulse width modulated pulse train needed to control the transistor power switch. A number of such start circuits are known in the art, but all of them have disadvantages where one desires to maintain the pulse generator isolated on the secondary side of the power supply. Maintaining isolation in a power supply between its primary winding and its secondary winding sides is necessary in many applications. In such supplies, the secondary side of the supply must be kept completely electrically isolated from the primary side. It is a common requirement, for example, that commercial isolated power supplies be able to withstand a 5,000 volt power surge without breakdown. Transformers having isolation between their primary and secondary windings, or opto-isolators, are used to provide such isolation. Transistor switches or other such semiconductor devices cannot be used for this purpose, since their breakdown voltage may only be of the order of 50-100 volts. A key problem in such isolated power supplies is how to power a pulse generator isolated from the primary side, especially when the power supply is being initially powered up when the pulse generator has not yet begun to regulate the output DC voltage.
One prior art solution to providing power to a pulse generator, while maintaining the isolation of the pulse generator means on the secondary side, was to couple a second power source across a second isolating transformer to the pulse generator. This is a complex and expensive solution to the problem.
Another solution was to position the pulse generator on the primary side and only generate the output DC voltage error signal on the secondary side of the power transformer. This error signal was then coupled across an isolator back to the primary side. The drawback of this solution is two fold. First, it is difficult to couple a DC level across an isolator, since either the error signal level needs to be converted into a pulse, as required by an isolation transformer, or a more expensive opto-isolator must be used. In addition, this solution requires that control circuitry be duplicated on both the primary and secondary sides of the power transformer.
A third and probably least desirable solution was to position the pulse generator on the primary side and have it monitor the primary winding voltage in an attempt to control the output voltage level. This solution presumes that the voltage on the output of the secondary will reflect or be a function of the voltage on the primary, but this is not necessarily the case, depending on the rate of change of the power supply's output load.
A recent prior art reference, U.S. Pat. No. 4,246,634, illustrates how difficult it is to successfully design an isolated power supply with a pulse generator on the secondary side of the isolated power supply. The invention disclosed in this reference purports to provide such an isolated supply but it fails in two respects. First, although the pulse generator is powered from a secondary winding, the voltage being monitored is not the output voltage. Rather, the monitored voltage is the same voltage that is generated by the secondary winding used to power the pulse generator. A different secondary winding couples energy to the output load. More importantly, the start circuit used in this prior art reference to initially power the pulse generator on power supply start-up couples power directly to the pulse generator. No high voltage isolation is provided. Once the pulse generator start circuit is operating normally, it is isolated from the pulse generator merely by a transistor that has been turned off. Such a circuit is susceptible, as described above, to high voltage noise spikes or other interference.
What is therefore needed is a means for powering an isolated pulse generator for a single ended switching power supply of the buck or boost type. The pulse generator must be initially powered on the secondary side by means of a start circuit on the primary side and thereafter powered from the secondary side once normal power supply operation is initiated.